Gas cluster ion beam (GCIB) processing apparatus are configured to generate cluster ion beams used for processing surfaces. In this description, gas clusters are defined as nano-sized aggregates of materials that would be gaseous under conditions of standard temperature and pressure. Such gas clusters typically consist of aggregates of from a few to several thousand molecules loosely bound to form the cluster. The clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams having controllable energy. Such ions each typically carry positive charges of q·e (where e is the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). Non-ionized clusters may also exist within a cluster ion beam. The larger sized cluster ions are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster ion energy. Consequently, the impact effects of large cluster ions are substantial, but are limited to a very shallow surface region. This makes cluster ions effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of conventional monomer ion beam processing.
Presently available cluster ion sources produce clusters ions having a wide distribution of sizes, N (where N=the number of molecules in each cluster ion—in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as a molecule and an ionized atom of such a monatomic gas will be referred to as a molecular ion—or simply a monomer ion—throughout this discussion). Many useful surface-processing effects can be achieved by bombarding surfaces with GCIBs. These processing effects include, but are not necessarily limited to, cleaning, smoothing, etching, doping, and film formation or growth.
To form gas clusters, a pressurized source gas is ejected through a nozzle into a vacuum, forming a supersonic gas jet. Cooling, which results from the expansion in the jet, causes gas in the jet to condense into clusters, each consisting of from several to several thousand weakly bound atoms or molecules. For many applications, gas clusters made from a noble gas like argon, or from a reactive gas like oxygen are useful. However, for certain applications such as semiconductor doping, reactive etching, or deposition of films having a particular desired stoichiometry, it is often desirable to form gas clusters from a gas mixture containing more than one gas in a known mixture. Such mixed gases may be purchased premixed from gas suppliers or may be mixed from separate pure source gases, immediately prior to use, using mass flow controllers or other mixing controls to achieve the required mixtures.
As with many beam processes, processing a surface with a GCIB requires careful control of the beam conditions and the beam dose. Beam dosimetry is typically done by integrating the charge-per-unit-surface that the ion beam delivers to the surface being processed. In the case of GCIB's, factors other than the beam dose can also influence the processing results. In addition to delivered dose, the average cluster ion charge state, the average cluster ion mass, and the average cluster ion energy of a GCIB can all influence the surface processing effects of a GCIB incident on a surface.
In applications requiring gas cluster ion beam processing using gas clusters formed from mixed gases, in addition to dose control and control of other beam parameters, it is often a requirement for successful processing that the mixture of gas species in the gas cluster ions be precisely known and hence it is generally necessary that the mixture or ratios of component gases of the source gas also be precisely known. For example, the precision of the doping and/or stoichiometry of deposited films is directly dependent on (along with other factors) the ratio of gas components in a source gas mixture. A mis-processed batch of semiconductor wafers, or other other GCIB processed products, can be a costly error.
Furthermore, it is often desirable to have a process check on the mixture of the source gases or on the constitution of the gas-cluster ions in order to assure that the correct gas mixture is being used and/or that the correct gas-cluster ion constitution results. However, it is often costly, or may be otherwise impractical, to perform a chemical analysis of a premixed gas bottle before use in a GCIB processing apparatus. Moreover, at other times, a gas mixture may be formed in the GCIB apparatus tool itself. In these situations, conventional GCIB processing apparatus are unable to control the flows of the individual process gases combined to form the gas mixture.
For these and other reasons, there is a need for improved GCIB processing apparatus and methods of controlling a composition of the gas mixture used to form a gas cluster ion beam.